During the industrial coating process, a wide variety of media are covered with different surface materials. For example, paper may be covered with starch solutions for improved heat resistance characteristics, and metal sheeting may be coated with paint or latex for aesthetic value or corrosion protection of oxidizing surfaces. The coating of materials on media is widely used in the industry, and improved, cost-effective apparatus, methods, and devices are continuously sought. The coating of liquids may utilize volatile solvents and require drying processes that create gas wastes requiring treatment. Apparatuses and methods for applying coating material in powder form to a medium do not suffer from the above shortcomings. Powders must adhere temporarily to the medium and be uniformly spread to prevent bumps or cause problems during post-treatment operations. Once applied to a medium, powders may require post-treatment operations such as baking to fix the powder permanently on the surface.
One of the known ways to adhere a powder to a surface without adding unnecessary agents or adhesives is by using the electrostatic adhering capacity of a charged stream of particles made from a powder suspended in a gas and placed in contact with a medium that has a different electrical energy or is grounded. The Law of Coulomb provides that electrostatic force felt by two bodies charged with the same polarity charge is a repulsive force, and the force felt by two bodies charged with opposite polarity is an attractive force. Once the powder particles in a stream are charged, either by removing or adding surface electrons, the particles are then drawn by the electromagnetic force to a grounded medium in proportion to Coulomb's Law. Another advantage of electrostatic charging of a stream of particles is the creation of repulsion forces between neighboring particles in the stream placed at equivalent energy to aid in the spatial distribution of the particles within the stream of particles. Additionally, charged particles are drawn by a stronger electrostatic force on a surface where other particles have not yet attached.
Electrostatic charges can be placed on a medium by contact electrification, triboelectric electrification, or physical rubbing of surfaces such as the friction of a balloon on a piece of clothing or the displacement of shoes over a carpet. Another way to create an electrical charge on an item is to circulate the item in a strong electrical field in excess of the breakdown strength of air, a field of such intensity that ionized particles are formed. These ions are collected on the surface of the item in the corona discharge zone around a conductor by moving the powder through the corona region. These particles exit the corona superficially charged with an ionic charge and are then vulnerable, due to their low mass, to electrostatic forces created by their charge. Particles of both conductive material and insulating material are vulnerable to corona charging. Nonconductive particles, since they are less likely to redirect the position of superficial ionic charges, are more likely to maintain their newly gained electrostatic charge.
What is known is the use of a high-level energy conductor located at the source of a stream of particles to ionize the powder or the use of a highly charged and dangerous conductive net structure placed in proximity to a medium. What is also known is the use of a chamber wherein the medium and the conductor are placed in contact with particles in the closed environment, or the use of an enclosure where ionized particles are collected after being placed in proximity to a conductor in a small enclosure before the ionized particle flow is directed onto a medium outside the enclosure. Drawbacks of these known technologies include the creation of corona discharges between the conductor surrounding low-level charge elements located in close proximity to the source of powder particles, the need to place the conductor in the path of the stream of particles, the creation of enclosed devices where high-level voltage must be managed, and distribution systems where the particles are not suspended in the air sufficiently enough to offer an optimal collection of the ions in the air. Although many of these devices are able to perform their intended functions in a workmanlike manner, none of them adequately addresses the combination of these drawbacks. What is needed is an apparatus able to adequately fluidize the particles from a powder source and place them in a particle stream, an apparatus where conductors are protected and offset from the particle stream, and an apparatus able to uniformly deposit the particles onto a medium. The present invention solves these and many other problems associated with currently available apparatuses for electrostatic coating.